This relates to a method of selective gas etching of a silicon nitride layer deposited on a semiconductive body of silicon.
It is well known that silicon nitride layers are of significance to semiconductor devices. They are used for various purposes, for example, for masking, for surface passivation, for isolation purposes or for manufacturing planar structures. When using silicon nitride layers in manufacturing structures, it is often necessary to remove certain areas from layers deposited on the entire surface of the structures to expose the semiconductive material e.g. for planar diffusion or bonding (contacting) purposes. This, however, calls for new etching techniques because, on one hand, silicon nitride layers in a buffered hydrofluoric acid require extremely long etching times, thus causing the photoresist layers of the masks to become damaged and, on the other hand, because the etching times in a concentrated hydrofluoric acid are short, but the concentrated acid destroys the material of the photoresist layers. Another disadvantage is the undercutting of the silicon nitride layers in cases where these layers are deposited on silicon dioxide layers.
One possible way of solving these problems resides in the use of hot phosphoric acid, as is described in the "Journal of the Electrochemical Society," 1967, pp. 869 to 872. According to this publication it is proposed to use a silicon dioxide (SiO.sub.2) etching mask. It is obvious that this process is expensive and circumstantial, and also the handling of boiling phosphoric acid is not unproblematic.
Recently a new etching technique has been introduced known as the so-called dry-etching technique. This comprises the gas etching of a silicon nitride layer in a plasma established by high-frequency glow discharge, with the advantage, besides the high etching rates, residing in that a photoresist layer can be used as the etch masking. According to U.S. Pat. No. 3,795,557, e.g. carbon tetra-fluoride as such or with the addition of oxygen is used as the gas atmosphere in which the silicon nitride layer is etched. When using the hitherto conventional gas mixtures, the direct etching of the silicon nitride layer without protecting the silicon surface of the semiconductor body lying therebelow, is not possible, because the difference between the etching rates of the silicon nitride and the silicon, in dependence upon the kind of gas mixture, is at about 1:3, so that the silicon also becomes etched. Attempts have been made to avoid this by depositing a silicon dioxide intermediate layer on the semiconductor surface for restricting the etching effect of the gas mixture. This measure, however, represents an additional step in the process and, consequently, does not contribute towards economizing the process.
In order to overcome this disadvantage it has already been proposed in the applicant's earlier German patent application No. P 26 14 977.9 to etch the silicon nitride layer in the reaction chamber of a high-frequency glow discharge with a power from 200 to 800 W in a gas mixture containing perfluoropropane as the fluorocarbon compound with the addition of a certain amount of oxygen by adjustment of a predetermined pressure. Perfluoropropane (C.sub.3 F.sub.8), however, is difficult to obtain, is costly, and moreover, the etching rates for silicon nitride are relatively low when using C.sub.3 F.sub.8.
As an inexpensively obtainable fluoride of carbon it is possible to use the aforementioned carbon tetrafluoride. This, however, then gives rise to the disadvantages mentioned hereinbefore in connection with this substance.